Golfballs generally include a spherical outer surface with a plurality of dimples formed thereon. Conventional dimples are circular depressions that reduce drag and increase lift. These dimples are formed where a dimple wall slopes away from the outer surface of the ball forming the depression.
Drag is the air resistance that opposes the golf ball's flight direction. As the ball travels through the air, the air that surrounds the ball has different velocities thus, different pressures. The air exerts maximum pressure at a stagnation point on the front of the ball. The air then flows around the surface of the ball with an increased velocity and reduced pressure. At some separation point, the air separates from the surface of the ball and generates a large turbulent flow area behind the ball. This flow area, which is called the wake, has low pressure. The difference between the high pressure in front of the ball and the low pressure behind the ball slows the ball down. This is the primary source of drag for golf balls.
The dimples on the golf ball cause a thin boundary layer of air adjacent to the ball's outer surface to flow in a turbulent manner. Thus, the thin boundary layer is called a turbulent boundary layer. The turbulence energizes the boundary layer and helps move the separation point further backward, so that the layer stays attached further along the ball's outer surface. As a result, a reduction in the area of the wake, an increase in the pressure behind the ball, and a substantial reduction in drag are realized. It is the circumference of each dimple, where the dimple wall drops away from the outer surface of the ball, which actually creates the turbulence in the boundary layer.
Lift is an upward force on the ball that is created by a difference in pressure between the top of the ball and the bottom of the ball. This difference in pressure is created by a warp in the airflow that results from the ball's backspin. Due to the backspin, the top of the ball moves with the airflow, which delays the air separation point to a location further backward. Conversely, the bottom of the ball moves against the airflow, which moves the separation point forward. This asymmetrical separation creates an arch in the flow pattern that requires the air that flows over the top of the ball to move faster than the air that flows along the bottom of the ball. As a result, the air above the ball is at a lower pressure than the air underneath the ball. This pressure difference results in the overall force, called lift, which is exerted upwardly on the ball. The circumference of each dimple is important in optimizing this flow phenomenon, as well.
By using dimples to decrease drag and increase lift, almost every golf ball manufacturer has increased their golf ball flight distances. In order to optimize ball performance, it is desirable to have a large number of dimples, hence a large amount of dimple circumference, which is evenly distributed around the ball. In arranging the dimples, an attempt is made to minimize the space between dimples, because such space does not improve aerodynamic performance of the ball. In practical terms, this usually translates into 300 to 500 circular dimples with a conventional sized dimple having a diameter that typically ranges from about 0.100 inches to about 0.180 inches.
When compared to one conventional size dimple, theoretically, an increased number of small dimples may enhance aerodynamic performance by increasing total dimple circumference. However, in reality small dimples are not always very effective in decreasing drag and increasing lift. This results at least in part from the susceptibility of small dimples to paint flooding. Paint flooding occurs when the paint coat on the golf ball partially fills the small dimples, and consequently decreases the dimple's aerodynamic effectiveness. On the other hand, a smaller number of large dimples also begin to lose effectiveness. This results from the circumference of one large dimple being less than that of a group of smaller dimples.
One attempt to improve the aerodynamics of a golf ball is to create a ridge-like polygon inside a non-circular dimple and near the center of the dimple, where the edges of the polygon are positioned below the un-dimpled surface of the hall. This approach is described in U.S. Pat. No. 6,315,686 B1 and U.S. patent application publication No. 2002/0025864 A1. The '686B1 and '864A1 references theorize that the polygonal ridges generate the turbulent boundary layer during low and intermediate ball velocities, and the non-circular dimples with the polygonal centers are used in conjunction with the conventional circular dimples on a golf ball. U.S. Pat. No. 4,869,512 also discloses the use of non-circular dimples with conventional circular dimples to improve aerodynamic performance of a golf ball. These non-circular dimples have shapes that include triangular, petal, oblong, and partially overlapping circles, among others. Additionally, U.S. Pat. No. 5,377,989 discloses non-circular isodiametrical dimples, wherein the dimples have an odd number of curved sides.
Another approach for improving the aerodynamics of a golf ball is suggested in U.S. Pat. No. 6,162,136, wherein a preferred solution is to minimize the land surface or undimpled surface of the ball to maximize dimple coverage. One way of maximizing the dimple coverage of the ball is to pack closely together circular dimples having various sizes, as disclosed in U.S. Pat. Nos. 5,957,786 and 6,358,161. In practice, the circular dimple coverage is limited to about 85% or less when non-overlapping dimples are used. Another attempt to maximize dimple coverage is to use polygonal dimples with polyhedron dimple surfaces, i.e., dimple surfaces constructed from planar surfaces, as suggested in a number of patent references including U.S. Pat. Nos. 6,290,615B1, 5,338,039, 5,174,578, 4,090,716, and 4,830,378, among others. Theoretically, higher dimple coverage is attainable with these polygonal dimples. However, it has been demonstrated that polygonal dimples with polyhedron dimple surfaces do not achieve performance improvements commensurate with their coverage improvements. It is believed that the linear edges of the polygonal dimples and the connecting sharp apices generate more drag than the curved edges of the circular dimples.
Hence, there remains a need in the art for a golf ball that has a high dimple coverage and superior aerodynamic performance.